Detection of ice and organics on an asteroidal surface

Ice on asteroid 24 Themis Two groups, both using the NASA Infrared Telescope Facility (IRTF) on Mauna Kea in Hawaii but independently, have obtained infrared spectra of the main-belt asteroid 24 Themis consistent with the widespread presence of a frosty coating containing water ice and organics. Although the presence of water on the surface of some asteroids had been inferred from the comet-like activity of several small asteroids, these are the first actual measurements of water and organics in the asteroid belt. The presence of surface ice is particularly surprising because of the relatively short lifetime that exposed ice has at this distance from the Sun — between the orbits of Mars and Jupiter. Recent evidence has blurred the line between comets and asteroids, although until now neither ice nor organic material had been detected on the surface of an asteroid. Here, the spectroscopic detection of water ice and organic material on the asteroid 24 Themis is reported. Water ice thus seems to be more common on asteroids than previously thought, and may be widespread in asteroidal interiors at smaller heliocentric distances than expected. Recent observations, including the discovery 1 in typical asteroidal orbits of objects with cometary characteristics (main-belt comets, or MBCs), have blurred the line between comets and asteroids, although so far neither ice nor organic material has been detected on the surface of an asteroid or directly proven to be an asteroidal constituent. Here we report the spectroscopic detection of water ice and organic material on the asteroid 24 Themis, a detection that has been independently confirmed 2 . 24 Themis belongs to the same dynamical family as three of the five known MBCs, and the presence of ice on 24 Themis is strong evidence that it also is present in the MBCs. We conclude that water ice is more common on asteroids than was previously thought and may be widespread in asteroidal interiors at much smaller heliocentric distances than was previously expected.